When Sylvia finds the elusive heron's nest and the heron with its mate, this influences the action of the story because Sylvia now has to make a decision. She has to decide whether to reveal the location of the heron to the hunter who wants to kill it and stuff it, or to keep its whereabouts secret.
At stake is the ten dollars the man is offering Sylvia and her grandmother for the location of the heron. At this time, this was a significant amount of money, and Sylvia and her grandmother are poor. The money would come in very handy, but on the other hand, Sylvia hates the idea of the white heron being killed.
It becomes clear to the hunter that Sylvia must know where the heron is, and both he and her grandmother want her to tell him how to find it:
But Sylvia does not speak after all, though the old grandmother fretfully rebukes her, and the young man's kind, appealing eyes are looking straight in her own.
Sylvia doesn't know exactly why she can't reveal the secret, but she knows it has to do with her memories of the white heron flying against the golden sky. She also recalls she and the white heron watching the sea together in the morning. She feels a kinship with the bird that she doesn't want to spoil. An innocent child of nature herself, she chooses not to betray the bird.